Generate FL/fl_config.h rather than FL/abi-version.h

... as discussed in fltk.coredev in thread "RFC: introduce public
config header <FL/fl_config.h>", see:
https://groups.google.com/g/fltkcoredev/c/xLCs1AIXMVo/m/MHZpQggzAQAJ

- Rename abi-version.h to fl_config.h, rename input files,
  update dependencies, .gitignore, CMake, configure and Makefiles.

- Include Cairo options in FL/fl_config.h

- Rename FLTK_USE_CAIRO to FLTK_HAVE_CAIROEXT for consistency.

- Include <FL/fl_config.h> in config.h and wherever necessary,
  fix include order (move FL/Fl.H to the top) and more.

- Move USE_X11 to fl_config.h and rename to FLTK_USE_X11

- Do not include <config.h> in Cairo demo program which is no
  longer required in Cairo programs since FLTK 1.4.0

CMake/android.cmake

@@ -31,8 +31,8 @@ configure_file(
 
 # create a custom abi file for this setup
 configure_file(
-  "${CMAKE_CURRENT_SOURCE_DIR}/abi-version.cmake.in"
-  "${CMAKE_CURRENT_BINARY_DIR}/AndroidStudio/FL/abi-version.h"
+  "${CMAKE_CURRENT_SOURCE_DIR}/fl_config.cmake.in"
+  "${CMAKE_CURRENT_BINARY_DIR}/AndroidStudio/FL/fl_config.h"
   @ONLY
 )
 

CMake/fl_create_example.cmake

@@ -116,6 +116,11 @@ macro (CREATE_EXAMPLE NAME SOURCES LIBRARIES)
   set_target_properties   (${TARGET_NAME} PROPERTIES OUTPUT_NAME ${NAME})
   target_link_libraries   (${TARGET_NAME} ${LIBRARIES})
 
+  # we must link all programs with fltk_cairo if option CAIROEXT is enabled
+  if (FLTK_HAVE_CAIROEXT)
+    target_link_libraries (${TARGET_NAME} fltk_cairo cairo)
+  endif ()
+
   if (FLTK_HAVE_CAIRO)
     fl_target_link_directories (${TARGET_NAME} PRIVATE "${PKG_CAIRO_LIBRARY_DIRS}")
   endif ()

CMake/install.cmake

@@ -34,7 +34,7 @@ install (DIRECTORY
   DESTINATION ${FLTK_INCLUDEDIR} USE_SOURCE_PERMISSIONS
   FILES_MATCHING
     PATTERN "*.[hH]"
-    PATTERN "abi-version.h" EXCLUDE
+    PATTERN "fl_config.h" EXCLUDE
 )
 
 install (DIRECTORY

CMake/options.cmake

@@ -76,7 +76,7 @@ set (PATH_TO_XLIBS)
 if ((NOT APPLE OR OPTION_APPLE_X11) AND NOT WIN32)
   include (FindX11)
   if (X11_FOUND)
-    set (USE_X11 1)
+    set (FLTK_USE_X11 1)
     list (APPEND FLTK_LDLIBS -lX11)
     if (X11_Xext_FOUND)
       list (APPEND FLTK_LDLIBS -lXext)
@@ -164,7 +164,7 @@ option (OPTION_CAIROEXT
 )
 
 set (FLTK_HAVE_CAIRO 0)
-set (FLTK_USE_CAIRO 0)
+set (FLTK_HAVE_CAIROEXT 0)
 
 if (OPTION_CAIRO OR OPTION_CAIROEXT)
   pkg_search_module (PKG_CAIRO cairo)
@@ -174,7 +174,7 @@ if (OPTION_CAIRO OR OPTION_CAIROEXT)
   if (PKG_CAIRO_FOUND)
     set (FLTK_HAVE_CAIRO 1)
     if (OPTION_CAIROEXT)
-      set (FLTK_USE_CAIRO 1)
+      set (FLTK_HAVE_CAIROEXT 1)
     endif (OPTION_CAIROEXT)
     add_subdirectory (cairo)